Field of the Invention
The present invention relates to an CVVT (Continuous Variable Valve Timing) apparatus for an engine, and, more particularly, to a technology for fixing a phase of an CVVT apparatus in a specific state in an operating region in which a valve timing is not actively adjusted.
Description of the Related Art
When an engine is driven in an Atkinson cycle, compression work may be reduced and thus fuel efficiency may be improved. An CVVT apparatus is utilized to realize the Atkinson cycle in a typical Otto-cycle engine.
That is, when the closing of an intake valve in a conventional Otto-cycle engine is delayed using an CVVT apparatus, compared to that in a typical engine, a compression stroke, in which energy for compressing air is consumed when a piston moves toward a top dead point, is practically decreased. Thus, the compression work of the engine may be relatively reduced compared to when the intake valve is not closed due to a delay, so that fuel efficiently may be improved.
However, since the CVVT apparatus is generally operated by hydraulic pressure produced using the power of the engine, it is difficult to actively control the CVVT apparatus when the engine is started.
In addition, when a valve timing is not actively controlled by the CVVT apparatus in the operating region during the idling of the engine, it is advantageous to fuel efficiency since energy consumption is rather small.
Accordingly, the CVVT apparatus is in a default state in which the CVVT apparatus is not actively controlled when the engine is started or idles.
In the engine equipped with the CVVT apparatus, which is configured to adjust the phase of an intake camshaft enough to realize the Atkinson cycle, when the default state is formed in which the CVVT apparatus is not actively controlled by hydraulic pressure, the CVVT apparatus is maintained in the most delayed state by resistance to driving of the intake camshaft. In this case, since the intake valve is delayed and closed in the most delayed state, there is a lack of compression pressure, resulting in the faulty starting of the engine and the poor combustion during the idling of the engine.
Therefore, it is necessary to fix the CVVT apparatus in the state, in which the intake valve timing of the typical Otto-cycle engine is realized, in the operating region, in which the CVVT apparatus is not actively controlled, such as in the operating region when the engine is started or idles.
The information disclosed in this Background section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.